The present invention relates to engines having a storage trap for oxides of nitrogen (NOx) in the exhaust system and is concerned with the periodic purging of such a trap of stored NOx gases and regenerating of the trap from sulphur poisoning by removal of oxides of sulphur (SOx) that accumulate in the trap.
In a lean burn spark ignition internal combustion engine used in a motor vehicle, a known method of reducing NOx emissions in the exhaust gases is to use a lean NOx trap which intercepts and stores NOx gases in batches from the oxidising atmosphere of the lean exhaust gases. The trap is purged periodically by exposing it to a reducing atmosphere, whereupon the stored NOx gases are released and are reduced to nitrogen in the presence of a catalyst before being discharged into the ambient atmosphere. If there is sulphur in the fuel, SOx will be present in the exhaust gases as well as NOx and the trap will also store the SOx gases which will in time poison the trap. To regenerate the trap from sulphur poisoning, the trap may, once again, be exposed to a reducing atmosphere but in this case the chemical reaction needs to take place at a higher temperature and must therefore either be carried out while the trap is heated or when the trap is already itself operating at a high temperature on account of the prevailing engine speed and load conditions.
A known method of exposing the trap to a reducing atmosphere involves changing the fuel calibration of the engine from a lean mixture to a rich mixture for a short period of time in order to switch the composition of the exhaust gases from excess oxygen to excess fuel. This method has the disadvantage that the operation of the engine is disturbed during the purge process which normally would cause a sudden change in the output torque of the engine and would be disconcerting to the driver of the vehicle. To avoid this, it necessary to take further complicated steps to suppress the sudden torque change so that the drive quality is not impaired.
The lean NOx trap is better positioned in the exhaust system at a substantial distance from the engine in order to ensure optimum operating temperature of the trap. A disadvantage of this, however, is that the duration of the rich operation of the engine during purging must take into account the transport and mixing of the resulting slug of rich exhaust gases with the lean exhaust gases along the exhaust pipe causing diffusion of the slug and diminishing the reducing atmosphere when the slug arrives at the trap.
Furthermore, because an oxidising catalytic converter must be provided upstream of the lean NOx trap for reducing the hydrocarbon emissions from the engine, the duration of the rich operation during purging must also take into account the oxidation by the catalytic converter of some of the excess fuel contained in the slug of rich exhaust gases before the remaining content of the slug can break through the oxidation catalytic converter. All these considerations have resulted in a relatively prolonged duration of the rich operation of the engine in order to ensure that a sufficient quantity of reducing exhaust gases can reach the lean NOx trap to purge the trap. This represents an increased penalty in the overall fuel consumption of the engine as the ratio of the durations of the rich and lean operations of the engine is increased.
With a view to mitigating the foregoing disadvantages, the present invention provides in accordance with a first aspect a method of purging a lean NOx trap that has a matrix of narrow flow passages and is arranged in the exhaust system of a lean burn engine, the method comprising the steps of providing a flow straightening matrix of narrow flow passages preceding the NOx trap matrix and separated from the NOx trap matrix by a narrow chamber, and periodically injecting reducing gases in bursts into the narrow chamber, each burst having sufficient mass and flow rate to fill the narrow chamber with the reducing gases and to displace the exhaust gases previously present in the narrow chamber into the narrow flow passages of the flow straightening matrix and of the trap without significantly mixing with the latter exhaust gases.
According to a second aspect of the invention, there is provided a lean burn engine having a lean NOx trap in the form of a matrix of narrow flow passages and means for injecting reducing gases upstream of the NOx trap matrix to purge the trap periodically, characterised in that a flow straightening matrix of narrow flow passages is arranged upstream of the NOx trap matrix and is separated from the trap matrix by a narrow chamber, and in that the means for injecting reducing gases upstream of the NOx trap matrix are operative to inject the reducing gases in bursts into the narrow chamber, each burst having sufficient mass and flow rate to fill the narrow chamber with the reducing gases and to displace the exhaust gases previously present in the narrow chamber into the narrow flow passages of the flow straightening matrix and of the trap without significantly mixing with the latter exhaust gases.
The invention allows the trap to be purged with a minimum of fuel wastage because the excess fuel injected for the purpose of purging is kept away to the extent possible from the surplus oxygen contained in the exhaust gases. Mixing of the gases is prevented on account of the narrow geometry of the chamber and of the flow passages. When the reducing gases are injected as a burst into the narrow chamber at a flow rate significantly higher than that of the exhaust gases flowing through the chamber, they rapidly fill the chamber forcing the exhaust gases into the narrow flow passages of the adjacent matrices on either side of the chamber and forming a slug of reducing gases in between the exhaust gases. This slug would interrupt the through flow of exhaust gases momentarily until the exhaust gas pressure in the upstream section of the exhaust system increases sufficiently to force its way back into the narrow chamber at which point the injected burst would be terminated. The result is that a wave of reducing gases will enter the lean NOx trap matrix and propagate along the length of the matrix without significantly mixing with the exhaust gases. The thickness of the wave will depend upon the mass of reducing gases that is injected in the time available before the through flow of exhaust gases forces its way back into the narrow chamber.
In one embodiment of the invention, the NOx trap matrix is coated not only with an NOx storage material but also with an oxygen storage material. In this case, the reducing gases can react exothermically with stored oxygen to release heat. By applying successive bursts of reducing gases at a repetition frequency which allows sufficient time between the bursts for sufficient exhaust gases to pass through to replenish the oxygen storage material with oxygen, one can heat the trap sufficiently to purge not only NOx but also any SOx that may have accumulated in the trap on account of the presence of sulphur in the fuel.
The reducing gases may be fuel vapour, a mixture of fuel vapour and air at richer than stoichiometric fuel-to-air ratio, or a mixture of fuel vapour and exhaust gases containing excess air at richer than stoichiometric fuel-to-air ratio. The rich fuel-to-air mixture may be reacted at the lean NOx trap matrix to form reducing gases such as carbon monoxide and hydrogen through partial oxidation and water-gas equilibrium reactions. The mixture may also be reacted externally before being injected into exhaust system, for example in a heated vessel which may be pressurised and may additionally contain a catalyst, a spark plug or a glow plug.
The flow straightening matrix may be a ceramic honeycomb of similar construction as that of the lean NOx trap matrix except that no coating need be applied to the surfaces of the narrow flow passages of the matrix.
The fuel vapour may be drawn from a vapour extraction system described in co-pending British Patent Application No. GB 9716156,6 for separating gasoline fuel into a lighter vapour fraction and a heavier liquid fraction. Alternatively or additionally, fuel vapour may be drawn from a vapour storage canister connected to the fuel storage tank of the engine. As a further alternative, liquid fuel may be injected under pressure into a heated vessel in which the fuel boils to form vapour and pressurises the vessel at the same time.
A vapour pump or compressor may be used to compress the reducing gases containing fuel vapour into a pressurised storage vessel from which it may be metered in bursts into the engine exhaust system in front of the lean NOx tray matrix by way of a fuel injector valve.